In a magnetic contactor, electric current is applied to an operation coil forming an electromagnet device to generate in a fixed iron core an attraction force to attract a movable iron core, thereby causing a movable contact to make a contact with or separate from a fixed contact. This enables the opening and closing of a circuit between a single-phase power supply or three-phase power supply and load equipment.
Conventionally, various proposals have been made for a coil drive circuit to be used in a magnetic contactor (see, for example, PTL 1 and PTL 2).
PTL 1 discloses a coil drive device for a magnetic contactor including: a semiconductor switching element to supply source voltage for the operation coil; a voltage detection circuit to detect the source voltage; a gain circuit to output an closing level signal based on the voltage detected by the voltage detection circuit and to output, after a predefined time has expired, a holding level signal based on the detected voltage, the holding level signal being higher than the closing level signal; a reference wave generation circuit to generate a sawtooth wave; a comparator to compare the sawtooth wave generated by the reference wave generation circuit with the closing level signal outputted by the gain circuit to output a closing pulsed signal with a constant period and to compare the sawtooth wave with the holding level signal after a predefined time has expired to output a holding pulsed signal having a smaller ON/OFF time ratio (also called duty ratio) than that of the closing pulsed signal; and a pulse output circuit to supply the closing pulsed signal and the holding pulsed signal for the semiconductor switching element.
In other words, PTL 1 discloses technology for exciting the coil with a large current during the circuit-closing control when a large attraction force is necessary due to a large gap between the iron cores of the electromagnet (in other words, the fixed contact and the movable contact are widely apart) and for minimizing the coil current to reduce the power consumption during the holding control when the contact can be maintained by exciting the operation coil with a relatively small current as the iron cores are attached together with no core gap.
PTL 2 discloses a circuit for controlling an electromagnet comprising a main current control element in series with the electromagnet winding, the main current control element acting initially as a switch to allow “pull-in” current to pass through the operation coil winding and subsequently as a current limiter to limit the current to a lower “hold” level. PTL 2 discloses that the circuit comprises a capacitor having associated diodes and/or switching components for connecting it across the winding once during change over from “pull-in” to “hold” to acquire a reverse charge thereon and again at switch off. As a result the capacitor can discharge in the operation coil winding to provide an electromagnetic force in opposition to the eddy currents sustained in the core after the main current control element is switched off. As a result the capacitor can discharge in the operation coil winding to provide a magnetizing force in a direction different from and in opposition to the magnetizing force of the eddy currents after the main current control element is switched off.
In other words, to shorten the release time of the electromagnet, i.e., the time for releasing the movable contact of the electromagnet from the hold state into the release state, PTL 2 describes technology to decrease the release time of the electromagnet by connecting a capacitor in parallel with the coil to charge the capacitor with the operation coil current after the electromagnet is turned off and allowing the charge to be discharged in the opposite direction to apply a magnetizing force in opposition to that of the electromagnet.